Designers and manufacturers of small portable computers ("lap-tops") are confronted with two major problems: the relatively short amount of time a lap-top can operate without external power, and size limitations that require trade-offs among typing efficiency, user comfort and portability.
Previous attempts at solving these problems include the provision of power packs containing the primary battery for the computer and most have some mechanism to swap a low battery for a charged one. However, despite new technology increasing the life of lap-top batteries, a single battery still only provides between one and five hours of power. To overcome the inconvenience of short battery life, several manufacturers have built lap-tops with capacity for two batteries within the case. The weakness of this approach is that it requires the user to forego a peripheral such as a floppy disk drive when using the secondary battery because the small size of the standard laptop case leaves little if any room for anything other than electronic devices and the primary battery.
The size of the lap-top case is also a limiting factor in incorporating a palm rest in front of the keyboard on which users ray rest their palms and wrists while typing. The smaller lap-top cases have very little surface area remaining after the keyboard has been installed Therefore in order to provide a palm rest, the manufacturer must choose between using a larger case or reducing the size of the keyboard. Using a larger case increases the weight of the lap-top and makes it more difficult to fit in a brief case. It may also add to the minimum operating weight. Reducing the size of the keyboard avoids these problems but introduces its own drawbacks. Whether the manufacturer combines several key functions into one key, eliminates certain keys and/or makes the keys themselves smaller, the user's typing becomes more error-prone. While palm rests make typing more comfortable and therefore more efficient, any benefit gained by having a palm rest on the lap-top is offset by the increase in key stroke errors. Despite their disadvantages, palm rests on lap-tops remain a desirable feature because they may help prevent discomfort and potential long term injury to the user.